I thought the same. I thought it was already a high risk to go into the building at all, because they weren`t sure the virus wasn`t already in the whole building. Still I think a shut-off of the system might have been a good idea - but maybe the system isn`t so easy to shut down - there is, after all, a reason why the buidling needs it)
Possibilities (no spoilers, only assumptions):
Considering the fact that Michelle and Almeda haven`t had the time to work their recent differences out, it might be a good "shock"-effect if Michelle (and the rest of the team in the building) would also die from the virus, but I am not sure if it would be contagious enough to allow them.
Regarding Chase:
I am not sure he will die this season. He has proven much more resistant and resourceful than most characters, so he might also be a replacement for Jack, in the long run, (= next season), if Kiefer Sutherland and the authors decide they need a real shock and kill of Jack! (Unfortunately, I don`t hink that the actor of Chase has the same presence as Sutherland)
Though it might also be used to make his death a bit harder for us viewers.
Mustrum Ridcully